Headed fasteners are used in a multitude of industrial assembly processes. In the aircraft industry, large quatities of flat head and dome head (hereinafter generically referred to as flush head) countersunk fasteners in the form of rivets and bolts are used in fabricating wings and other parts of an aircraft. Such fasteners are inserted into predrilled countersunk holes in the pieces being fabricated. Rivets are tightened by forming a head on the end of the rivet remote from the countersunk headed end and bolts are tightened by installing a collar or a nut on the end of the bolt remote from the countersunk headed end. If the countersunk head of the headed fastener does not precisely fit into the countersink region of the predrilled hole (i.e., the funnel-shaped enlargement formed at the insertion surface of the workpiece), rather than the edge of the countersunk head being flush with the insertion surface after installation, either a part of the head of the fastener will protrude from the insertion surface or the head will be recessed in the surface. Both protruding and recessed installed fastener heads are undesirable in aircraft parts, particularly when the fastener insertion surface is to form an external aircraft surface, because sharp protrusions and recesses both create aerodynamic drag. While protruding fasteners can be machined to remove excessive protruding material, this is a time consuming and, thus, expensive procedure. Recess fastener heads are even more undesirable because they cannot be economically eliminated. Unfortunately, the tolerances of most currently available standard size countersunk headed fasteners are such that it is rare that a fastener will precisely fit a predrilled hole. In most instances, the head of such fasteners either protrudes from, or is recessed into, the insertion surface of the part being fabricated. While fastener tolerances can be tightened in order to reduce the number of installed fasteners having protruding or recessed heads, tightening tolerances significantly increase the cost of fasteners and, consequently, the cost of the part being fabricated. Hence, this is an undesirable resolution of this problem.
Fortunately, while precise fasteners cannot be economically manufactured, machine tools that can drill precisely sized holes in a part are available. The invention is designed to allow this feature of modern manufacturing equipment to be used advantageously. More specifically, the holes in which flat headed countersunk fasteners are to be installed have a cylindrical region for receiving the shank of the fastener and a countersink region for receiving the countersunk head of the fastener. After a cylindrical region sized to accept the shank of a fastener is drilled, a constant depth countersink is created on one end of the cylindrical hole. The depth of the countersink is set so that the "average" size fastener is flush with the insertion surface after a fastener is installed. As discussed above, frequently, the tolerances of fasteners result in the heads of some fasteners protruding from the insertion surface and the heads of other fasteners being recessed into the insertion surface. One way of avoiding this problem is to vary the depth of the countersink region of the fastener hole to compensate for tolerance variations. In order to accomplish this result, it is necessary to know the elevation of the head of the fastener to be installed in a particular hole. More specifically, it is necessary to know the elevation of the head above the point where the shank (e.g., the fixed diameter section) will intersect the cylindrical region of the hole in which the fastener will be installed. The invention is directed to providing an apparatus that measures this elevation.